Destroy All Planets (1968)

Starring Kojiro Hongo, Toru Takatsuka, Carl Craig, Michiko Yaegaki, Mari Atsumi
Directed by Kenji Yuasa
(actor & director credits courtesy IMDB.com)

Invading space aliens discover the Earth is protected by the flying turtle monster Gamera, but try to exploit his weaknesses by kidnapping a pair of young children.

This Gamera sequel is probably the weakest of the series I've yet seen, featuring less monster action and trying to make up for it by replaying lengthy battle scenes from the previous films, which should have been shortened considerably.  Although the final confrontation with a giant octopus-like creature isn't half bad, the lead-up to that sequence is a bit yawn-inducing, as there's not much of interest once the movie's focus shifts to the young boys' captivity aboard the drab alien spaceship.  By casting American Carl Craig in the role of one of the boys, the film was clearly aimed at both the Japanese and U.S. markets, an approach that would be repeated in the next sequel, Attack Of The Monsters.